Heat Resistant Titanium Alloy Sheet Excellent in Cold Workability and A Method of Production of the Same

ABSTRACT

The present invention provides a heat resistant titanium alloy sheet excellent in cold workability having high temperature strength characteristics better than JIS Type 2 pure titanium and having a cold workability and high temperature oxidation resistance equal to or better than that of JIS Class 2 pure titanium comprising, by mass %, 0.3 to 1.8% of Cu, 0.18% or less of oxygen, 0.30% or less of Fe, and, as needed, at least one of Sn, Zr, Mo, Nb, and Cr in a total of 0.3 to 1.5%, and the balance of Ti and less than 0.3% of impurity elements and a method of production characterized by performing the final annealing at 650 to 830° C. or performing the hot-rolled sheet or coil annealing or intermediate annealing at 650 to 830° C. and the final annealing after cold working at 600 to 650° C.

TECHNICAL FIELD

The present invention relates to a heat resistant titanium alloy sheet excellent in cold workability and a method of production of the same, more particularly relates to a heat resistant titanium alloy sheet excellent in cold workability suited for exhaust system parts of two-wheeled and four-wheeled vehicles and other applications where characteristics in a high temperature range and cold workability are required and a method of production of the same.

BACKGROUND ART

The exhaust system of a two-wheeled or four-wheeled vehicle (hereinafter referred to as an “automobile”) is comprised of an exhaust manifold, exhaust pipe, muffler, and other parts. To enable it to withstand high temperature exhaust gas or to cope with complicated shapes, stainless steel excellent in corrosion resistance, high temperature strength, workability, etc. is being made considerable use of.

However, in recent years, pure titanium, which has a corrosion resistance superior to stainless steel, is light in weight, is excellent in workability as well, has a small heat expansion coefficient, is superior in heat fatigue characteristics, and is excellent in terms of aesthetic design due to its unique color and impression, has started to be used in the exhaust systems of some automobiles, in particular for the mufflers. The amount used has been rapidly increasing.

A muffler is the final part in an exhaust system. The exhaust gas there has been cooled to a certain extent. Further, it is frequently used for the outside pipe exposed to the outside air for design purposes. For this reason, pure titanium, which is not that high in high temperature strength, can also be used for muffler applications. Rather, the excellent cold workability of pure titanium is being utilized for working the metal into complicated shapes.

Such pure titanium parts, like stainless steel parts, are mainly made of cold rolled annealed thin-gauge sheet which is bent, press formed, drawn, and enlarged in holes (bored) or is bent and welded to form welded pipe or is cold worked in various ways to form it into the desired shape for use.

Such pure titanium thin-gauge sheet is generally produced by the following process. That is, VAR (vacuum arc remelting) or EBR (electron beam remelting) or another remelting process is used to form an ingot, this is hot forged or break-down rolled to form a slab, then this is hot rolled to form a hot rolled strip and further descaled, then cold rolled to form a cold rolled strip. Alternatively, this is cut to produce cut sheet products.

Note that during these processes, the metal may be annealed as required before the cold rolling (after the hot rolling) or in the middle of the cold rolling. Further, the final cold rolled strip is also generally annealed.

On the other hand, the exhaust pipe or exhaust manifold near the engine is often exposed to a high temperature. If trying to use a titanium material for the inside and outside pipes of a muffler of an automobile with a high exhaust temperature, it would be necessary to use thick pure titanium to reinforce the strength or use an alloy excellent in high temperature strength such as Ti-3Al-2.5V alloy.

However, using thick pure titanium has the problem of detracting from the special feature of titanium of its light weight, Further, an alloy having 3% or so of Al such as a Ti-3Al-2.5V alloy is poor in cold workability. Therefore, there were the problems that the cold rollability to thin-gauge sheet of the material when producing pipe for an exhaust system part was impaired or the cold formability such as pipe bending dropped.

To solve the above problems, Japanese Patent Publication (A) No. 2001-234266 discloses an invention relating to a titanium alloy for muffler use to which 0.5 to 2.3 mass % of Al has been added, that is, a titanium alloy for an exhaust system part superior to even pure titanium in heat resistance and oxidation resistance and having a cold rollability equal to that of pure titanium.

DISCLOSURE OF THE INVENTION

However, the invention described in the above Japanese Patent Publication (A) No. 2001-234266 does indeed have an excellent cold rollability equal to that of the JIS Class 2 pure titanium made much use of for mufflers, but as shown in Table 1 and FIGS. 2 to 4 of that publication, compared with JIS Class 2 pure titanium, the yield strength is high and the ductility is low, so when the sheets or the pipes produced using the same are bent, enlarged, reduced, enlarged in hole size (bored), or otherwise secondarily worked, a further higher cold workability is sought.

Further, in ships etc. as well, there is a strong need for reducing the weight of the exhaust system parts. A titanium material excellent in both workability and high temperature strength has therefore been strongly sought.

The present invention was made taking note of the above situation and has as its object the provision of heat resistant titanium alloy sheet excellent in cold workability having high temperature strength characteristics better than JIS Class 2 pure titanium and having cold workability and high temperature oxidation resistances equal to or better than those of JIS Class 2 pure titanium and a method of production of the same.

To solve the above problems, the present invention has the following means as its framework:

(1) A heat resistant titanium alloy sheet excellent in cold workability characterized by comprising, by mass, 0.3 to 1.8% of Cu, 0.18% or less of oxygen, 0.30% or less of Fe, and the balance of Ti and less than 0.3% of impurity elements.

(2) A heat resistant titanium alloy sheet excellent in cold workability as set forth in the above (1), characterized in that said titanium alloy sheet further contains at least one or more of Sn, Zr, Mo, Nb, and Cr in a total of 0.3 mass % to 1.5 mass %.

(3) A method of production of heat resistant titanium alloy sheet excellent in cold workability produced by the steps of remelting, hot rolling, hot-rolled sheet or coil annealing, cold rolling, intermediate annealing, final annealing, etc., said method of production of a titanium alloy sheet characterized by adjusting the ingredients at said remelting to the composition of ingredients as set forth in the above (1) or (2) and performing said final annealing at 650 to 830° C. in temperature range.

(4) A method of production of heat resistant titanium alloy sheet excellent in cold workability produced by the steps of remelting, hot rolling, hot-rolled sheet or coil annealing, cold rolling, intermediate annealing, final annealing, etc., said method of production of a titanium alloy sheet characterized by adjusting the ingredients at said remelting to the composition of ingredients as set forth in the above (1) or (2), performing said hot-rolled sheet or coil annealing or said intermediate annealing at 650 to 830° C. in temperature range, and performing said final annealing at 600 to 650° C. in temperature range.

BEST MODE FOR WORKING THE INVENTION

The inventors carefully evaluated the effects of ingredient elements in the high temperature strength, oxidation resistance, and cold workability of titanium so as to solve the above problems and as a result discovered that if adding a certain amount of Cu to the titanium, it is possible, without impairing the cold workability or oxidation resistance, to remarkably improve the high temperature strength in the temperature range in which automobile exhaust system members etc. are used, i.e.; about 500 to about 700° C. The present invention was completed based on this epoch making discovery.

Now, in the invention described in claim 1 (hereinafter referred to as “the present invention (1)”), the alloy is comprised of, by mass, 0.3 to 1.8% of Cu, 0.18% or less of oxygen, 0.30% or less of Fe, and the balance of Ti and less than 0.3% of impurity elements.

If adding Cu to titanium, it enters into solid solution in the α-phase in as much as 1.5%. This solid solution Cu, like Al, has the effect of increasing the high temperature strength by solid solution strengthening. On the other hand, in Al-added titanium and Cu-added titanium, a remarkable difference appears in the cold workability.

That is, if cold working Al-added titanium, not only does the slip deformation responsible for deformation become harder to occur, but also the occurrence of twinning deformation, the main reason for the high workability of titanium, is suppressed, the yield strength becomes higher, and the ductility falls. As a result, the cold workability falls.

However, with Cu-added titanium, while the slip deformation is suppressed by the solution strengthening, the occurrence of twinning deformation is not impaired at all. The result is like pure titanium. As a result, a low yield strength and ductility on a par with Type 2 pure titanium are maintained. Of course, this effect is an effect expressed when the twinning deformation is the main deformation mechanism. Like with Al, oxygen, which as an effect of suppression of the occurrence of twinning, has to be limited to the upper limit value for active twinning, that is, 0.18% or less.

Here, the amount of addition of Cu is given an upper limit of 1.8% because if Cu is added over this, a Ti₂Cu phase will be formed in a large amount and the cold workability will be impaired. Further, the amount of addition of Cu is given a lower limit of 0.3% because to sufficiently bring out a high temperature strength, the Cu has to be added in an amount of 0.3% or more.

Note that content of Fe has to be 0.30% or less. Fe is an element stabilizing the β-phase and causes the formation of the β-phase from room temperature to the high temperature range. If the content of Fe is 0.30% or less, the amount of formation of the β-phase is slight, but if more than this is added, the amount of the β-phase increases, Cu, an element which easily concentrates at the β-phase, will concentrate there heavily, and the amount of solid solution in the α-phase required for improving the high temperature strength will fall. Therefore, to suppress the formation of an excessive β-phase, Fe has to be made 0.30% or less.

However, nitrogen, carbon, Ni, Cr, Al, Sn, Si, hydrogen, and other elements normally contained in a titanium material as impurity elements and other elements may be contained without problem if the total does not impair the workability, i.e., is less than 0.3%.

Further, the high temperature oxidation resistance, an important characteristic to be possessed by a heat resistant material like high temperature strength, is not impaired at all even if Cu is added.

In the alloy of the present invention (1), from the viewpoint of the workability, the content of oxygen is preferably 0.10% or less. This is because, with this range of oxygen amount, the occurrence of twinning is further promoted and the workability is further improved. Oxygen has almost no effect on the high temperature strength, so even if limiting the oxygen to 0.10% or less, the high temperature characteristics are not impaired at all.

This type of effect can be manifested further by limiting the content of oxygen to 0.06% or less. That is, in the alloy of the present invention (1), if the content of oxygen is 0.06% or less, the effect of the present invention is exhibited the strongest.

Next, the present invention described in claim 2 (hereinafter referred to as “the present invention (2)”) will be explained. In the present invention (2), there is provided the alloy of the present invention (1) further containing at least one or more of Sn, Zr, Mo, Nb, and Cr in a total of 0.3 mass % to 1.5 mass.

This is to try to further improve the high temperature strength of the alloy of the present invention (1) and to try to further improve the high temperature oxidation resistances. Sn, Zr, Mo, Nb, and Cr all enter the α-phase to a certain extent in solid solution and overlap with the Cu to raise the high temperature strength. Further, simultaneously, the high temperature oxidation characteristics are also improved.

However, the amount of addition has to be, in total, 0.3% or more. This is because if not the above amount of addition, an improvement in the high temperature strength and an improvement in the high temperature oxidation resistance cannot be obtained. Further, the amount of addition has to be, in total, not more than 1.5%. This is because these elements have the effect of promoting the precipitation of Ti₂Cu. If added in a large amount, the amount of production of Ti₂Cu increases and therefore the workability is impaired. However, if the total is 1.5% or less, this effect is small.

The present invention described in claim 3 or 4 (hereinafter referred to as “the present inventions (3) and (4)”) relates to a method of production of thin-gauge sheet used in large amounts in exhaust systems of automobiles. That is, the present invention (3) is a method of production of thin-gauge sheet having titanium alloy ingredients of the present invention (1) or (2) produced by the steps of remelting, hot rolling, and cold rolling, said method of production of a titanium alloy sheet of the present invention (1) or (2) characterized in that the final annealing is performed at 650 to 830° C. in temperature range.

This condition aims at increasing the amount of solid solution Cu as much as possible from the viewpoint of the workability and the high temperature strength. Of course, even if performing annealing or other heat treatment outside of this temperature range, if the ingredients are those of the present invention (1) or (2), the effects of the present invention are sufficiently exhibited, but if performing the annealing in this temperature range, the effect of the present invention can be further enhanced.

That is, 650 to 830° C. is a temperature range where the amount of production of Ti₂Cu is small and the amount of solid solution Cu in the α-phase becomes larger. By annealing in this temperature range, the high temperature strength can be particularly raised.

Note that if Ti₂Cu is produced during the cooling after the annealing, it is pointed out that the targeted annealing effect ends up being impaired, but Ti₂Cu precipitates very slowly. With the cooling rate of the extent of air cooling or furnace cooling, not enough Ti₂Cu is produced for the annealing effect to be impaired.

Further, if once annealing at 650 to 830° C. in temperature range, even if later cold working the alloy and again annealing it at less than 650° C. in temperature, since Ti₂Cu precipitates slowly, within the actual heat treatment time, almost no Ti₂Cu will be produced and therefore the large amount of Cu in solid solution in the α-phase can be maintained.

That is, if performing the annealing before the final cold rolling (hot-rolled sheet or coil annealing or intermediate annealing) at 650 to 830° C. in temperature range, even if performing the final annealing after the cold rolling at less than 650° C. in temperature, the large amount of Cu in solid solution in the α-phase can be maintained. This method of production is used by the present invention described in claim 4. However, at less than 600° C. in temperature, strain becomes difficult to remove and softening becomes difficult, so sufficient cold workability cannot be obtained, so this should be avoided.

EXAMPLES Example 1

VAR (vacuum arc remelting) was used to remelt the titanium material of each composition shown in Table 1. This was hot forged to form a slab which was then heated to 860° C., then hot rolled by a hot continuous rolling mill to a strip of a thickness of 3.5 mm.

This hot rolled strip was continuously annealed with air cooling at 720° C.×2 minutes (hot-rolled coil annealing), then the oxide scale was removed by shot blast and pickling, then the strip was cold rolled to a strip of a thickness of 1 mm. After this, the strip was vacuum annealed with furnace cooling at 680° C.×4 hours (final annealing). A tensile test piece was taken in parallel with the rolling direction and was used for tensile tests at room temperature, 550° C., 625° C., and 700° C. The strength characteristics were evaluated by the 0.2% proof stress or yield stress (hereinafter referred to as “0.2% yield strength”), while the workability was evaluated by the elongation value at room temperature. Further, a 30 mm×30 mm square test piece was heat treated at 700° C.×200 hours in the air and measured for increase in weight due to oxidation. The results of these evaluations are shown together in Table 1.

TABLE 1 Room 700° C., temperature Room 550° C. 625° C. 700° C. 200 h 0.2% yield temperature 0.2% yield 0.2% yield 0.2% yield oxidation Test Cu Al Fe O strength elongation strength strength strength weight increase no. (mass %) (mass %) (mass %) (mass %) (MPa) (%) (MPa) (MPa) (MPa) (mg/cm²) Remarks 1 — — 0.05 0.18 275 39.5 60 21 8 3.02 Conv. mat. 2 — 1.1 0.05 0.13 310 28.9 105 62 20 2.98 Conv. mat. 3 — 2.1 0.05 0.08 403 25.2 126 81 37 2.94 Conv. mat. 4 0.2 — 0.05 0.08 205 40.6 65 28 11 2.97 Comp. ex. 5 0.4 — 0.05 0.08 203 41.8 101 80 31 3.01 Inv. (1), (3) 6 0.8 — 0.05 0.08 207 41.0 116 87 35 2.96 Inv. (1), (3) 7 1.6 — 0.05 0.08 211 40.3 133 95 41 3.02 Inv. (1), (3) 8 2.0 — 0.05 0.08 220 31.8 135 97 44 3.00 Comp. ex. 9 0.8 — 0.15 0.08 202 40.5 118 89 36 3.03 Inv. (1), (3) 10 0.8 — 0.26 0.08 225 40.1 116 88 40 2.99 Inv. (1), (3) 11 0.8 — 0.33 0.08 232 37.2 103 75 18 3.05 Comp. ex. 12 1.1 — 0.06 0.12 251 38.3 118 90 38 2.99 Inv. (1), (3) 13 1.1 — 0.05 0.16 279 36.2 120 88 37 2.96 Inv. (1), (3) 14 1.1 — 0.05 0.20 301 30.5 120 87 37 2.98 Comp. ex. 15 1.5 — 0.05 0.16 280 35.8 130 97 41 3.08 Inv. (1), (3) 16 1.0 — 0.04 0.07 207 42.5 115 88 36 3.01 Inv. (1), (3) 17 1.0 — 0.04 0.04 195 47.5 114 86 35 2.96 Inv. (1), (3) 18 1.0 — 0.03 0.02 189 48.3 115 87 34 3.00 Inv. (1), (3)

In Table 1, Test No. 1 is an example of JIS Class 2 commercially pure titanium, while Test Nos. 2 and 3 are examples of alloys to which Al has been added in an extent of 1 to 2%. Test No. 1 has an elongation at room temperature of as much as 39.5% and a sufficient cold workability, but the 0.2% yield strength at high temperatures is poor being only 60 MPa at 550° C., 21 MPa at 625° C., and 8 MPa at 700° C., i.e., the high temperature strength is insufficient.

As opposed to this, Test Nos. 2 and 3 to which Al are added have 0.2% yield strengths at 550° C., 625° C., and 700° C. all far above that of the pure titanium of Test No. 1, i.e., high high-temperature strength is achieved, the elongation at room temperature is 30% or less, and the cold workability is insufficient.

In this way, if a small amount of Al is added, the high temperature strength is improved, but the cold workability falls. The market demand for a titanium alloy satisfying both requirements is not been achieved by this.

As opposed to this, Test Nos. 5, 6, 7, 9, 10, 12, 13, 15, 16, 17, 18 representing examples of the present invention (1) produced by the method described in the present invention (3) all have high elongations at room temperature of at least 35% and have 0.2% yield strengths at 550° C., 625° C., and 700° C. of at least 100 MPa, at least 80 MPa, and at least 30 MPa. Both an excellent cold workability and high high-temperature strength are achieved, i.e, the effect of the present invention is sufficiently exhibited.

In particular, in Test Nos. 5, 6, 7, 9, 10, 16, 17, and 18 where the content of oxygen is 0.10% or less, 40% or higher elongations at room temperature are obtained, that is, the effects of the present invention (1) are sufficiently exhibited. In particular, in Test Nos. 17 and 18 where the content of oxygen is 0.06% or less, 45% or higher extremely high elongations at room temperature are obtained. The effect of the present invention (1) is most strongly exhibited. Note that the amount of increase in weight due to oxidation during heat treatment in the air at 700° C. for 200 hours was, in the examples of the present invention, about the same level as that of the pure titanium of Test No. 1 and the Al-added titanium alloys of Test Nos. 2 and 3.

However, in Test No. 4, while a high 40.6% room temperature elongation was obtained, the 0.2% yield strengths at 550° C., 625° C., and 700° C. were 100 MPa, 80 MPa, and 30 MPa or less, that is, a sufficient improvement was not achieved in the high temperature strength. Further, Test No. 11 also exhibited at a high 37.2% room temperature elongation, but the 0.2% yield strengths at 625° C. and 700° C. were 80 MPa and 30 MPa or less, i.e., the improvement in the high temperature strength was not sufficient.

The reason is that, in Test No. 4, the amount of addition of Cu is less than the lower limit value of 0.3% of the present invention, so the amount of Cu in solid solution required for improving the high temperature strength was insufficient. In Test No. 11, the content of Fe, the β-phase stabilization element, is over the upper limit value of 0.30% of the present invention, so the amount of the β-phase increases, Cu concentrates there heavily, and the amount in solid solution in the α-phase required for improvement of the high temperature strength falls.

Further, in Test Nos. 8 and 14, the high temperature strengths were sufficiently high, but the room temperature elongations were both not more than 35% or were considerably lower values compared with JIS Class 2 pure titanium. This is because, in Test No. 8, Cu is added over the upper limit value of 1.8% of the present invention, so a large amount of the Ti₂Cu phase is produced and the cold ductility is impaired. In Test No. 14, the content of oxygen is over the upper limit value of 0.18% of the present invention, so the twinning deformation is suppressed and the cold deformability drops.

In the above way, the titanium alloy sheet comprised of the elements defined in the present invention is provided with excellent cold workability and high temperature strength and, further, has high temperature oxidation characteristics on a par with pure titanium, but if deviating from the amounts of alloying elements defined in the present invention, both the cold workability and the high temperature strength cannot be achieved.

Example 2

VAR (vacuum arc remelting) was used to remelt the titanium material of each composition shown in Table 2. This was hot forged to form a slab which was then heated to 860° C., then hot rolled by a hot continuous rolling mill to a strip of a thickness of 3.5 mm.

This hot rolled strip was continuously annealed with air cooling at 720° C.×2 minutes (hot-rolled coil annealing), then the oxide scale was removed by shot blast and pickling, then the strip was cold rolled to a strip of a thickness of 1 mm. After this, the strip was vacuum annealed with furnace cooling at 680° C.×4 hours (final annealing). A tensile test piece was taken in parallel with the rolling direction and was used for tensile tests at room temperature and 700° C.

The strength characteristics were evaluated by the 0.2% yield strength, while the workability was evaluated by the elongation value at room temperature. Further, a 30 mm×30 mm square test piece was heat treated at 700° C.×200 hours in the air and measured for increase in weight due to oxidation. The results of these evaluations are shown together in Table 2.

TABLE 2 Test Cu Sn Zr Mo Nb Cr Fe no. (mass %) (mass %) (mass %) (mass %) (mass %) (mass %) (mass %) 19 0.8 1.3 — — — — 0.05 20 0.8 1.7 — — — — 0.05 21 0.8 — 1.3 — — — 0.05 22 0.8 — 1.8 — — — 0.05 23 0.8 — — 1.4 — — 0.05 24 0.8 — — 1.7 — — 0.05 25 0.8 — — — 1.4 — 0.05 26 0.8 — — — 1.8 — 0.05 27 0.8 — — — — 1.2 0.05 28 0.8 — — — — 1.6 0.05 29 0.8 0.5 0.7 — — — 0.05 30 0.8 0.5 — 0.7 — — 0.05 31 0.8 0.5 — — 0.6 — 0.05 32 0.8 0.5 — — — 0.5 0.05 33 0.8 — 0.5 0.5 0.3 — 0.05 34 0.8 — — 0.5 0.3 0.5 0.05 35 0.8 — — 0.2 1.0 — 0.05 36 0.8 0.5 — 0.8 — 0.5 0.05 37 0.8 0.8 — — — 0.8 0.05 38 1.1 — — — — 0.8 0.06 39 1.1 — — — 0.5 — 0.06 40 1.1 — — 1.0 — — 0.06 41 1.1 — 0.9 — — — 0.06 42 1.1 0.9 — — — — 0.06 43 1.0 — — — — 0.33 0.04 44 1.0 — — — 0.4 — 0.04 45 1.0 — — 0.4 — — 0.04 46 1.0 — 0.4 — — — 0.04 47 1.0 0.5 — — — — 0.04 48 1.0 — — — — 0.2 0.04 49 1.0 — — — 0.2 — 0.04 50 1.0 — — 0.2 — — 0.04 51 1.0 — 0.2 — — — 0.04 52 1.0 0.2 — — — — 0.04 Room 700° C., temperature Room 700° C. 200 h 0.2% yield temperature 0.2% yield oxidation Test O strength elongation strength weight increase no. (mass %) (MPa) (%) (MPa) (mg/cm²) Remarks 19 0.08 303 38.0 42 2.84 Inv. (2), (3) 20 0.08 310 37.0 45 2.82 Comp. ex. 21 0.08 302 39.2 48 2.85 Inv. (2), (3) 22 0.08 318 33.0 45 2.79 Comp. ex. 23 0.08 301 39.3 48 2.84 Inv. (2), (3) 24 0.08 318 32.8 48 2.80 Comp. ex. 25 0.08 299 38.7 46 2.81 Inv. (2), (3) 26 0.08 321 31.5 45 2.74 Comp. ex. 27 0.08 298 36.8 49 2.86 Inv. (2), (3) 28 0.08 320 31.2 50 2.86 Comp. ex. 29 0.08 299 38.8 44 2.79 Inv. (2), (3) 30 0.08 297 37.7 46 2.79 Inv. (2), (3) 31 0.08 295 36.6 44 2.74 Inv. (2), (3) 32 0.08 290 37.9 45 2.81 Inv. (2), (3) 33 0.08 302 36.0 44 2.77 Inv. (2), (3) 34 0.08 305 37.5 47 2.77 Inv. (2), (3) 35 0.08 310 37.7 42 2.74 Inv. (2), (3) 36 0.08 325 29.8 50 2.76 Comp. ex. 37 0.08 327 30.5 49 2.80 Comp. ex. 38 0.12 293 38.2 43 2.87 Inv. (2), (3) 39 0.12 272 37.8 45 2.84 Inv. (2), (3) 40 0.12 290 39.3 47 2.81 Inv. (2), (3) 41 0.12 292 40.1 44 2.80 Inv. (2), (3) 42 0.12 288 37.5 46 2.79 Inv. (2), (3) 43 0.07 285 37.9 44 2.89 Inv. (2), (3) 44 0.07 271 37.5 46 2.88 Inv. (2), (3) 45 0.07 288 39.5 48 2.87 Inv. (2), (3) 46 0.07 295 40.2 45 2.87 Inv. (2), (3) 47 0.07 289 37.3 47 2.89 Inv. (2), (3) 48 0.07 283 37.7 37 2.95 Inv. (1), (3) 49 0.07 270 38.0 38 2.96 Inv. (1), (3) 50 0.07 286 39.3 38 2.94 Inv. (1), (3) 51 0.07 292 41.0 39 2.95 Inv. (1), (3) 52 0.07 285 38.1 39 2.94 Inv. (1), (3)

In Table 2, Test Nos. 19, 21, 23, 25, 27, 29, 30, 31, 32, 33, 34, and 35 representing examples of the present invention produced by the method described in the present invention (3) all had high elongations at room temperature of over 35%. Further, compared with Test No. 6 comprised of the same amounts of Cu, Fe, and oxygen, the 0.2% yield strengths at 700° C. became at least 7 MPa higher. The effect of addition of Sn, Zr, Mo, Nb, and Cr alone or combined was therefore exhibited.

Further, the increases in weight due to oxidation during heat treatment in the air at 700° C. for 200 hours were also smaller than that of Test No. 6—less than 2.90 mg/cm² in each case, i.e., an improvement in high temperature oxidation resistance was also achieved. This was due to the effect of addition of Sn, Zr, Mo, Nb, or Cr alone or in combination.

Test Nos. 20, 22, 24, 26, 28, 36, 37 exhibited 0.2% yield strengths at 700° C. higher than Test No. 6 and increases in weight due to oxidation during heat treatment in the air at 700° C. for 200 hours smaller than Test No. 6. The high temperature strengths and the high temperature oxidation characteristics were improved, but the room temperature elongations were less than 35% in each case, i.e., the workabilities ended up being impaired.

This is because the total of the amounts of addition of the one or more of Sn, Zr, Mo, Nb, and Cr was over the upper limit value of 1.5% of the present invention, so the precipitation of Ti₂Cu was promoted and the workability was impaired.

Test Nos. 38 to 42 are examples of the present invention (2) comprised of the alloy of Test No. 12 to which Sn, Zr, Mo, Nb, and Cr are further added. Since the amounts of addition were suitable, high room temperature elongations of 35% or more, 0.2% yield strengths at 700° C. of over that of Test No. 12, and high temperature oxidation characteristics during heat treatment in the air at 700° C. for 200 hours were achieved.

Test Nos. 43 to 52 are examples of the alloy of Test No. 16 to which Sn, Zr, Mo, Nb, and Cr are added. Test Nos. 43 to 47 to which suitable amounts were added as prescribed in the present invention (2) achieved high room temperature elongations of 35% or more, high temperature strengths (0.2% yield strengths at 700° C.) higher than Test No. 16 by more than 5 MPa, and high high-temperature oxidation characteristics (high temperature oxidation characteristics during heat treatment in the air at 700° C. for 200 hours) were achieved. On the other hand, Test Nos. 48, 49, 50, 51, and 52 in which the amounts of addition of Sn, Zr, Mo, Nb, and Cr were less than the 0.3% prescribed by the present invention (2) had margins of improvement of the high temperature strength of at most 3 MPa, and the margin of improvement of the high temperature oxidation characteristics was little.

Example 3

Sheets were taken from the intermediate products when producing the materials of Test No. 6 of Table 1 and Test Nos. 29, 34 and 44 of Table 2, that is, hot rolled strips of 3.5 mm thickness. These were hot-rolled sheet annealed under the conditions shown in Tables 3 to 6, the oxide scales were removed by shot blast and pickling, then these were cold rolled to 1 mm thick strips. After this, each strip was cold-rolled sheet annealed under the conditions described in Tables 3 to 6 (final annealing). A tensile test piece was taken in parallel to the rolling direction and was used for tensile tests at room temperature and 700° C.

The strength characteristics were evaluated by the 0.2% yield strength, while the workability was evaluated by the elongation value at room temperature. Further, a 30 mm×30 mm square test piece was heat treated at 700° C.×200 hours in the air and measured for increase in weight due to oxidation. The results of these evaluations are shown together in Tables 3 to 6.

TABLE 3 Room 700°C., Hot-rolled Cold-rolled temperature Room 700° C. 200 h sheet sheet 0.2% yield temperature 0.2% yield oxidation Test annealing annealing strength elongation strength weight increase no. conditions conditions (MPa) 1%) (MPa) (mg/cm²) Remarks 53 720° C., 2 min, 580° C., 6 h, 218 40.0 31 2.98 Inv. (1) air cooling furnace cooling 54 720° C., 2 min, 630° C., 4 h, 209 40.3 35 2.98 Inv. (1), (4) air cooling furnace cooling 55 720° C., 2 min, 680° C., 4 h, 207 41.0 35 2.96 Inv. (1), (3) air cooling furnace cooling 56 720° C., 2 min, 780° C., 30 min, 205 42.0 34 2.98 Inv. (1), (3) air cooling furnace cooling 57 720° C., 2 min, 810° C., 5 min, 200 42.3 34 2.95 Inv. (1), (3) air cooling air cooling 58 720° C., 2 min, 850° C., 3 min, 198 42.5 31 2.96 Inv. (1) air cooling air cooling 59 630° C., 10 min, 630° C., 4 h, 207 40.8 32 2.99 Inv. (1) air cooling furnace cooling 60 630° C., 10 min, 680° C., 4 h, 209 40.5 35 2.95 Inv. (1), (3) air cooling furnace cooling 61 630° C., 10 min, 780° C., 30 min, 207 41.0 36 3.00 Inv. (1), (3) air cooling furnace cooling 62 630° C., 10 min, 810° C., 5 min, 201 41.0 34 2.99 Inv. (1), (3) air cooling air cooling 63 630° C., 10 min, 850° C., 3 min, 197 42.8 31 3.01 Inv. (1) air cooling air cooling 64 850° C., 2 min, 630° C., 4 h, 207 42.2 31 2.95 Inv. (1) air cooling furnace cooling 65 850° C., 2 min, 680° C., 4 h, 208 40.5 36 2.93 Inv. (1), (3) air cooling furnace cooling 66 850° C., 2 min, 780° C., 30 min, 208 41.2 36 2.98 Inv. (1), (3) air cooling furnace cooling 67 850° C., 2 min, 810° C., 5 min, 201 42.3 35 2.98 Inv. (1), (3) air cooling air cooling 68 850° C., 2 min, 850° C., 3 min, 190 43.3 32 3.00 Inv. (1) air cooling air cooling

TABLE 4 Room 700° C., Hot-rolled Cold-rolled temperature Room 700° 200 h sheet sheet 0.2% yield temperature 0.2% yield oxidation Test annealing annealing strength elongation strength weight increase no. conditions conditions (MPa) (%) (MPa) (mg/cm²) Remarks 69 720° C., 2 min, 630° C., 4 h, 302 37.7 46 2.82 Inv. (2), (4) air-cooling furnace cooling 70 720° C., 2 min, 680° C., 4 h, 299 38.8 44 2.80 Inv. (2), (3) air-cooling furnace cooling 71 720° C., 2 min, 780° C., 30 min, 290 38.7 47 2.82 Inv. (2), (3) air-cooling furnace cooling 72 720° C., 2 min, 810° C., 5 min, 285 39.5 46 2.80 Inv. (2), (3) air-cooling air-cooling 73 720° C., 2 min, 850° C., 3 min, 281 39.6 40 2.79 Inv. (2) air-cooling air-cooling

TABLE 5 Room 700° C., Hot-rolled Cold-rolled temperature Room 700° C. 200 h sheet sheet 0.2% yield temperature 0.2% yield oxidation Test annealing annealing strength elongation strength weight increase no. conditions conditions (MPa) (%) (MPa) (mg/cm²) Remarks 74 630° C., 10 min, 630° C., 4 h, 311 36.5 40 2.84 Inv. (2) air-cooling furnace cooling 75 680° C., 5 min, 630° C., 4 h, 308 37.4 46 2.81 Inv. (2), (4) air-cooling furnace cooling 76 720° C., 2 min, 630° C., 4 h, 305 37.5 47 2.78 Inv. (2), (4) air-cooling furnace cooling 77 810° C., 2 min, 630° C., 4 h, 298 38.1 46 2.79 Inv. (2), (4) air-cooling furnace cooling 78 850° C., 2 min, 630° C., 4 h, 290 38.3 41 2.81 Inv. (2) air-cooling furnace cooling

Table 3 shows the results of tests on materials of the same composition as in Test No. 6. Regardless of the conditions of the hot-rolled sheet annealing, Test Nos. 55, 56, 57, 60, 61, 62, 65, 66, and 67 involving final annealing, that is, cold-rolled sheet annealing, at 650 to 830° C. in temperature range all gave high room temperature elongations of over 40% and high 0.2% yield strengths at 700° C. of over 34 MPa. The oxidation resistances were also on the level of pure titanium.

In this way, by applying the method described in the present invention 3, it is possible to produce products featuring all of room temperature workability, high temperature strength, and high temperature oxidation resistances.

Further, Test No. 54 had a temperature of the final annealing, that is, the cold-rolled sheet annealing, of 630° C. This was outside the range of conditions prescribed in the present invention (3), but a high room temperature elongation of over 40%, a high 0.2% yield strength at 700° C. of over 34 MPa, and oxidation resistances on a par with pure titanium were exhibited. This was because the annealing before the cold rolling, that is, the hot-rolled sheet annealing, was conducted at 650 to 830° C. in temperature range, so the effects of the present invention (4) were exhibited.

Note that Test Nos. 53, 58, 59, 63, 64, 68 all gave high room temperature elongations of over 40% and high 0.2% yield strengths a 700° C. of over 30 MPa, but compared with the invention examples, the high temperature strengths became somewhat lower. The reason is as follows:

Test No. 53 involved the annealing before cold rolling, that is, the hot-rolled sheet annealing, performed at the 650 to 830° C. temperature range prescribed in the present invention (4), but the final annealing, that is, the cold-rolled sheet annealing, was conducted at less than the 600° C. prescribed in the present invention (4), so the margin of improvement of the high temperature strength ended up becoming somewhat small. Test No. 58 had a final annealing, that is, a cold-rolled sheet annealing, outside of the temperature range prescribed by the present invention (3) or (4), so the margin of improvement of the high temperature strength ended up becoming somewhat smaller.

Test Nos. 59, 63, 64, and 68 had annealing before the cold rolling, that is, the hot-rolled sheet annealing, performed outside the 650 to 830° C. temperature range prescribed in the present invention (4) and had final annealing, that is, cold-rolled sheet annealing, outside the temperature range prescribed in the present invention (3), so the margin of improvement of the high temperature strength became somewhat small.

Now, Table 4 shows the results of tests on materials of the same composition as Test No. 29. The cold-rolled and annealed sheets produced by the method of present invention (3) or (4) (Test Nos. 69 to 72) all gave high room temperature elongations of over 35%, high 0.2% yield strengths at 700° C. of over 44 MPa, and excellent high temperature oxidation resistance.

However, Test No. 73 which involved final annealing, that is, the cold-rolled sheet annealing, performed outside of the temperature range prescribed in the present invention (3) or (4) had a 0.2% yield strength at 700° C. somewhat lower compared with the examples of Test Nos. 69 to 72.

Further, Table 5 shows the results of tests on materials of the same composition as Test No. 34. The cold-rolled and annealed sheets of Test Nos. 75 to 77 produced by the method described in the present invention (4) all gave high room temperature elongations of over 35%, high 0.2% yield strengths at 700° C. of over 46 MPa, and excellent high temperature oxidation resistances.

However, in Test Nos. 74 and 78 involving annealing before the cold-rolling, that is the hot-rolled sheet annealing, performed outside of the 650 to 830° C. temperature range prescribed in the present invention (4) and involving final annealing, that is, cold-rolled sheet annealing, performed outside of the temperature range prescribed in the present invention (3), the 0.2% yield strengths at 700° C. became somewhat lower compared with the examples of Test Nos. 75 to 77.

Further, Table 6 shows the results of tests on materials of the same composition as Test No. 44. Test No. 80 produced by the method described in the present invention (3) and Test No. 81 produced by the method described in the present invention (4) both gave high room temperature elongations equal to Test No. 44, high 0.2% yield strengths at 700° C., and excellent high temperature oxidation resistances.

TABLE 6 Room 700° C., Hot-rolled Cold-rolled temperature Room 700° C. 200 h sheet sheet 0.2% yield temperature 0.2% yield oxidation Test annealing annealing strength elongation strength weight increase no. conditions conditions (MPa) (%) (MPa) (mg/cm²) Remarks 80 810° C., 2 min, 700° C., 4 h, 268 39.2 45 2.85 Inv. (2), (3) air-cooling furnace cooling 81 810° C., 2 min, 640° C., 4 h, 275 37.0 48 2.88 Inv. (2), (4) air-cooling furnace cooling

INDUSTRIAL APPLICABILITY

The titanium alloy sheet of the present invention can be particularly utilized for parts of an exhaust system of two-wheeled and four-wheeled automobiles, that is, the exhaust manifold, exhaust pipe, muffler, and other parts used for the discharge route of burned exhaust gas. 

1-4. (canceled)
 5. A heat resistant thin gauge, cold rolled titanium alloy sheet used in an exhaust system excellent in cold workability and high temperature strength characterized by consisting of, by mass %, 0.3 to 1.8% of Cu, 0.18% or less of oxygen, 0.05% or more to 0.30% of Fe, and the balance of Ti and less than 0.3% of impurity elements, said titanium alloy steel consisting essentially of an α phase, said titanium alloy sheet having a room temperature elongation of at least 35%. 